The invention relates to optical data storage, and particularly to high-density optical data storage involving the recording of data by burning or otherwise creating tiny pits into a data storage record medium using a relatively strong, pulsed focused laser beam, and reading of the data record using a relatively weaker continuous focused laser beam.
Optical devices have the potential for storage and retrieval of data at densities and rates far in excess of those possible using known magnetic techniques. In magnetic data storage, density is limited, inter alia, by the necessity of relatively large spacing between adjacent tracks of data in order to maintain the required magnetic separation. To the extent that a laser beam can be focused to a spot of serveral microns or less and accurately located on a data storage record and maintained in focus during operation, track-to-track separation is significantly less that that required for magnetic based systems.
Accordingly, high-density optical data storage requires extreme accuracy in the location of a focused laser beam, both in recording the data and in reading it. Slight variations in the distance between a data record and a laser source producing a converging beam can move the focal point of the beam off the data record resulting in an enlarged spot on the record which can lead to inaccurate results in both recording or reading data. Similarly, relative lateral shifting of the moving data record with respect to the laser beam during recording or reading can cause a beam to record or read at the wrong location. Vibrations or other eccentric movements from mechanical components or even dimensional changes in the data record medium due to temperature and humidity require that means be provided to maintain the laser beam in focus on the data record and at the proper location.
Present day devices using laser optics employ a rotating mechanism to carry data on a disk past a read/write laser beam. In order to maintain the necessary focus and position accuracies, the disks must be formed with precision and all moving parts must be carefully made to avoid, to the extent possible, extraneous motion. These systems are expensive to make and operate and pragmatic considerations require that system densities be compromised to some extent.
Since real systems are never perfect and some unwanted movements will always be experienced, some type of compensating mechanism must be deployed in optical systems if high density storage is to be realized.
Prior to the present invention, maintaining a laser beam in focus and on target has been accomplished by detection devices and servo motors to correct the relative positions of the laser and the data record in response to sensed inaccuracies in these positions. While this approach has worked well enough to permit commercialization of audio and video optical players, it has yet to produce an acceptable system for computer data storage.
The prior art does not contemplate, in the context of optical data storage, a recording and reading system which premits mechanical inaccuracy or other factors giving rise to transverse shifting and slight variations in separation between the laser source and the data record, while still providing the required accuracy for high density operation without the use of expensive, complex servo systems.